1. Field of the Invention
The present invention relates to an active matrix circuit fabricated using thin film insulated gate type semiconductor devices (thin film transistors or TFTs) formed on an insulating substrate (in this specification, the “insulating substrate” generally means a material having an insulating surface and, unless otherwise defined, it implies not only an insulating material such as glass but also a material such as a semiconductor or metal having an insulator layer formed thereon). More particularly, the present invention relates to a monolithic active matrix circuit which includes peripheral circuits for driving it formed on the same substrate. A monolithic active matrix circuit according to the present invention is suitable for matrix type displays such as liquid crystal displays and plasma displays.
2. Description of the Related Art
As shown in FIG. 5, a monolithic active matrix circuit is comprised of an active matrix circuit area, a source driver, and a gate driver which are formed using substantially the same process. In an active matrix circuit, TFTs are used as switching elements for liquid crystal cells. In order to supplement the electrostatic capacity of the liquid crystal cells, auxiliary capacities are provided in parallel with the liquid crystal cells. The peripheral circuits such as the source driver and gate driver are constituted by a shift register and switching elements which must be capable of operating at high speed. Taking this into consideration, monolithic active matrix circuits are constructed using a crystalline semiconductor (e.g., polycrystalline silicon). Further, in order to suppress power consumption, the peripheral circuits are constructed using complementary circuits (CMOS). Such techniques are described in Japanese unexamined patent publication (KOKAI) No. H1-289917 in which switching TFTs in an active matrix circuit and a TFT that constitutes a shift register of a peripheral circuit are described as having substantially the same sectional structure.
However, TFTs as the switching elements in an active matrix circuit and TFTs in CMOS circuits such as a shift register do not operate in the same way. For example, for a TFT as a switching element, a high reverse bias voltage (a negative voltage if the TFT is of the N-channel type) is applied to the gate electrode. On the other hand, principally, no reverse bias voltage is applied to a TFT in a CMOS logic circuit. Further, the operating speed of the former may be one percent or less of the speed required for the latter.
As described above, it has been considered undesirable to employ the same structure to build TFTs for which operating conditions and required characteristics are greatly different.